The present invention relates to a method of preparing a composite sintered body having inner and outer portions fitted with each other, which body is used as various machine elements such as sprockets, gears and cams.
Hitherto, there have been proposed methods of preparing a sintered body having inner and outer portions fitted with each other. For example, JP-B-62-35442 discloses a method of preparing a sintered body, in which method the carbon content of an inner powder compact is greater than that of an outer powder compact by at least 0.2 wt % and the inner and outer powder compacts fitted with each other are sintered. With this, the bonding strength between the inner and outer portions of the sintered body is improved. JP-B-63-15961 discloses another method of preparing a sintered body, in which method the carbon content of an inner powder compact is greater than that of an outer powder compact by at least 0.2 wt % and at least 50 wt % of iron powder of at least one of inner and outer powder compacts is a reduced iron powder. With this, a sintered body having an improved bonding strength between the inner and outer portions is produced with a low cost. According to JP-B-62-35442, the carbon content having a great influence on the hardness and mechanical strength of the sintered body is limited to a certain range. According to JP-B-63-15961, the carbon content is limited to a certain range and the iron powder is limited to a certain type. Thus, according to these publications, the mechanical property of each of the inner and outer portions of the sintered body is restricted. Thus, such mechanical property does not always meet the demand.